U.S. patent application number 12/847982 was filed with the patent office on 2012-02-02 for grating for multiple discrete wavelengths of raman scattering.
Invention is credited to Alexandre M. Bratkovski, Min Hu, Huei Pei Kuo, Jingjing Li, Zhiyong Li, Fung Suong Ou, Michael J. Stuke, Michael R.T. Tan, Shih-Yuan Wang, Wei Wu.
Application Number | 20120026493 12/847982 |
Document ID | / |
Family ID | 45526423 |
Filed Date | 2012-02-02 |
United States Patent
Application |
20120026493 |
Kind Code |
A1 |
Stuke; Michael J. ; et
al. |
February 2, 2012 |
GRATING FOR MULTIPLE DISCRETE WAVELENGTHS OF RAMAN SCATTERING
Abstract
Systems and methods employ a layer having a pattern that
provides multiple discrete guided mode resonances for respective
couplings of separated wavelengths into the layer. Further, a
structure including features shaped to enhance Raman scattering to
produce light of the resonant wavelengths can be employed with the
patterned layer.
Inventors: |
Stuke; Michael J.; (Palo
Alto, CA) ; Tan; Michael R.T.; (Menlo Park, CA)
; Bratkovski; Alexandre M.; (Mountain View, CA) ;
Hu; Min; (Sunnyvale, CA) ; Kuo; Huei Pei;
(Cupertino, CA) ; Li; Jingjing; (Palo Alto,
CA) ; Li; Zhiyong; (Redwood City, CA) ; Ou;
Fung Suong; (Palo Alto, CA) ; Wang; Shih-Yuan;
(Palo Alto, CA) ; Wu; Wei; (Palo Alto,
CA) |
Family ID: |
45526423 |
Appl. No.: |
12/847982 |
Filed: |
July 30, 2010 |
Current U.S.
Class: |
356/301 |
Current CPC
Class: |
G01N 21/658 20130101;
G01N 21/65 20130101 |
Class at
Publication: |
356/301 |
International
Class: |
G01J 3/44 20060101
G01J003/44 |
Claims
1. A system comprising: a layer having a grating pattern that
provides a plurality of discrete guided mode resonances for
respective couplings of a plurality of separated wavelengths into
the layer; and a structure that is adjacent to the layer and
includes features shaped to enhance Raman scattering that produces
light of the wavelengths.
2. The system of claim 1, wherein the grating pattern corresponds
to a transform of a coupling distribution having peaks at the
separated wavelengths.
3. The system of claim 1, wherein the grating pattern is such that
of each of the separated wavelengths is coupled into the layer from
an entire area of the grating pattern.
4. The system of claim 1, wherein the system comprises a monolithic
chip including the layer and the structure.
5. The system of claim 1, wherein each of the separated wavelengths
corresponds to a wavelength of light that results from Raman
scattering from a different analyte.
6. The system of claim 1, wherein each of the separate wavelengths
corresponds to a different wavelength of light that results from
Raman scattering from an analyte.
7. The system of claim 1, further comprising: a light source
positioned to illuminate the structures; and a detector position to
receive light coupled into the first layer.
8. The system of claim 1, wherein the detector comprises a system
selected from a group consisting of: a detector having a detection
band that includes the wavelengths; a plurality of detectors having
detection bands respectively corresponding to the wavelengths; and
a dispersive element that separates the wavelengths spatially and a
position sensitive detector positioned to separately measure the
wavelength separated by the dispersive element.
9. The system of claim 1, wherein the features are at least
partially positioned to be within an evanescent field of the guided
mode resonances.
10. A method comprising: exposing a system to a sample, wherein the
system includes: a grating pattern that provides a plurality of
discrete guided mode resonances for respective couplings of a
plurality of separated wavelengths into the layer; and a structure
that is adjacent to the layer and exposed to the sample, wherein
the structure includes features shaped to enhance Raman scattering
that produces light of the wavelengths; illuminating the system
with an excitation beam; and measuring light in the guided mode
resonances to detect one or more analytes.
11. The method of claim 10, wherein the grating pattern is such
that each of the separated wavelengths is coupled into the layer
from an entire area of the grating pattern.
12. The method of claim 10, wherein each of the separate
wavelengths corresponds to a different wavelength of light that
results from Raman scattering from an analyte.
13. The method of claim 10, wherein each of the separated
wavelengths corresponds to a wavelength of light that results from
Raman scattering from a different analyte from among a set of
targeted analytes.
14. The method of claim 13, further comprising generating in
response to measuring light a signal indicating that one of the
targeted analytes was detected.
15. The method of claim 13, further comprising separately measuring
the wavelengths to identify which of the target analytes are in the
sample.
Description
BACKGROUND
[0001] Raman scattering generally refers to the inelastic
scattering of photons. When light scatters from an atom or
molecule, a fraction of the photons induces a transition to or from
an excited state of the atom or molecule, which produces scattered
photons having a different frequency from the frequency of incident
photons. The frequencies of the Raman scattered photons are
characteristic of the atoms or molecules from which the photons
scatter, which permits spectral analysis to identify chemical
analytes. However, the fraction of photons scattered by Raman
scattering is generally small, e.g., about 1 per 10.sup.7
elastically scattered photons, and methods for enhancing the signal
associated with Raman scattering have been developed. In
particular, Surface Enhanced Raman Scattering (SERS) enhances the
Raman scattering through interactions of scattered photons with
rough metal surfaces or nanoparticles. Such enhancement is believed
to result from resonances in localized surface plasmons interacting
with photons and analytes. Methods and systems for further
enhancing the Raman scattering signal and/or improving chemical
analysis based on Raman scattering are sought.
BRIEF DESCRIPTION OF THE DRAWINGS
[0002] FIG. 1 illustrates a system in accordance with an embodiment
of the invention using a line grating having parameters that vary
to produce resonances at multiple light frequencies.
[0003] FIG. 2 illustrates a system in accordance with an embodiment
of the invention using a two-dimensional grating pattern with
parameters that vary to produce resonances at separated frequencies
of light.
[0004] FIG. 3 is a graph of a desired coupling strength as a
function of the wavelength of the light coupled into waveguide
layer.
[0005] FIG. 4 illustrates use of a grating system such as
illustrated in FIGS. 1 and 2 in a chemical analysis system.
[0006] Use of the same reference symbols in different figures
indicates similar or identical items.
DETAILED DESCRIPTION
[0007] In accordance with an aspect of the invention, a SERS system
can employ a guided mode resonance (GMR) grating with a pattern
that is optimized for multiple separated wavelengths. In
particular, instead of having a constant period or a chirped
period, the grating has parameters that vary as necessary to
provide resonant couplings for a set of discrete wavelengths. The
resonant wavelengths can all be different wavelengths associated
with the same analyte, e.g., the same molecule or atom, for use in
a system having enhanced selectivity in the detection of a single
analyte. Alternatively, the wavelengths can be associated with
different analytes for a system capable of detecting a number of
different analytes. The pattern required to achieve the desired
performance can be determined through a suitable transform such as
a Fourier transform of the desired coupling strengths to produce a
grating pattern that achieves the desired coupling strengths.
[0008] FIG. 1 illustrates a grating system 100 in accordance with
an embodiment of the invention based on a line grating pattern. In
some embodiments, system 100 may be employed in a spectroscopy
apparatus used for detection of one or more specific analytes in a
sample or the surrounding environment system 100. System 100 could
also be used in a sensor system adapted to detect one particular
analyte or any analyte from a targeted set of analytes. In
particular, system 100 includes a SERS layer 110 that would
generally be positioned to come into contact or close proximity to
atoms or molecules of one or more analytes. SERS layer 110 may, for
example, be the top layer of system 100 and may be exposed to the
air of the surrounding environment. Alternatively, SERS layer 110
may be in a channel through which a gaseous or liquid sample flows
or resides. SERS layer 110 generally includes features that
interact electromagnetically with photons or analytes to enhance
Raman scattering of photons from the analytes. As mentioned above,
such enhancement is believed to result from interactions with
resonances of surface plasmons in conductive features of SERS layer
110. SERS layer 110 in the illustrated embodiment includes
particles, e.g., gold or silver particles having sizes on the order
of several or tens of nanometers. In the illustrated embodiment,
particles in SERS layer 110 can be fabricated and then applied
system 100 so that the particles will be at least partly in an
evanescent field of guided mode resonances of a GMR layer 120.
Alternatively, SERS layer 110 can be fabricated on or overlying GMR
layer 120 to position SERS features in the evanescent field of
guided mode resonances.
[0009] GMR layer 120 includes a grating pattern that varies across
the area of system 100 as required to provide guided mode
resonances for a discrete set of wavelengths within GMR layer 120.
Guided mode resonances, which are sometimes referred to as "leaky
resonances," are understood in the art to correspond to resonances
excited in a waveguide by a phase-matching element such as a
diffraction grating. In general, a signal (e.g., light) incident on
GMR layer 120 will be strongly coupled into a resonance mode in GMR
layer 120 only under specific circumstances, e.g., when specific
conditions on the direction, the wavelength, and possibly the
polarization of the incident light are satisfied. In a system where
the incident direction of the light signal is fixed, the condition
for guided mode resonance in GMR layer 120 is that the incident
light has one of the discrete wavelengths .lamda..sub.1 to
.lamda..sub.N for an integer N equal to or greater than 2, e.g.,
2.ltoreq.N.ltoreq.5 or more.
[0010] FIG. 1 illustrates an embodiment of GMR layer 120 in which a
grating pattern and a waveguide are made of the same material,
e.g., a layer of a dielectric material such as silicon nitride.
Alternatively, GMR layer 120 can include layers of different
materials, for example, a layer in which a grating pattern is
formed and a layer that acts as a waveguide. Also, the grating
pattern in GMR layer 120 can be formed using regions of solid
materials having different refractive index, instead of using air
gaps formed in a solid layer as shown in FIG. 1.
[0011] The grating pattern of FIG. 1 particularly includes a series
of grooves 122 of depth t1, which leaves a thickness t2 of GMR
layer 120 beneath grooves 122. In general, thickness t2 can be 0,
so that grooves 122 pass through GMR layer 120. Grooves 122 leave
stripes 124 of thickness t1+t2 and width W, and the combined widths
of a stripe 124 and a groove 122 is referred to herein as the
period D of the grating pattern. The ratio W/D of the width W of a
stripe to the period D defines the duty cycle of the period
containing the stripe. The period D and width W of stripes 124 in
GMR layer 120 can be smaller than the wavelength of the signal
light. In accordance with an aspect of the invention, parameters,
particularly the period D, the width W, or the duty cycle W/D of
the grating pattern of GMR layer 120 varies across the area of
system 100 in a manner that provides guided mode resonances in GMR
layer 120 for a target incidence angle and the desired wavelengths
.lamda..sub.1 to .lamda..sub.N. Further, the coupling into the
guided mode resonances can be provided across the entire area of
the grating pattern, instead of in a patchwork fashion where
different separate areas of the grating pattern each couple
incident signal light into a corresponding only one of the guided
modes.
[0012] The grating pattern of FIG. 1 being based on grooves 122 may
provide couplings to the guide mode resonances that are dependent
on the polarization of incident light. In particular, light having
a linear polarization parallel or perpendicular to grooves 122 may
experience different couplings into waveguide layer 120. In many
applications, the polarization difference is not critical, for
example, when the polarization of incident light is fixed and
anticipated in the design of the grating pattern. However, a
two-dimensional grating may be employed to reduce or eliminate
polarization dependence.
[0013] FIG. 2 shows an embodiment of a grating system 200 in which
a GMR layer 220 has a two-dimensional grating pattern. In the
illustrated embodiment, the grating pattern is formed using air gap
regions 222 in GMR layer 220, but a two-dimensional grating pattern
could alternatively be formed using regions of solid material
having different refractive indexes. Regions 222, which are square
or rectangular in FIG. 2, could alternatively have any desired
shape. Also, the grating pattern in GMR layer 220 could be formed
in a layer of material that is different from the material used as
a waveguide within GMR layer 220. The two-dimensional grating
pattern of FIG. 2 could have periods and duty cycles along one
direction that are the same as or that differ from the periods and
duty cycles along a perpendicular direction. In accordance with an
aspect of the current invention, the period and duty cycle of the
grating pattern of GMR layer 220 varies across the area of GMR
layer 220 in order to provide guided mode resonances for a set of
discrete wavelengths .lamda..sub.1 to .lamda..sub.N.
[0014] System 200 as shown in FIG. 2 also illustrates an SERS layer
210 that is made of discrete metal regions formed on or overlying
GMR layer 220. Such regions may enhance Raman scattering from
nearby analyte atoms or molecules through plasmon interactions.
Raman scattering can be further enhanced through interactions with
the evanescent field that results outside of GMR layer 220 when GMR
layer 220 contains light in a guided mode resonance.
[0015] FIGS. 1 and 2 illustrate embodiments of the invention in
which GMR layers 120 and 220 are fabricated on a carrier substrate
130. Carrier substrate 130 may have a refractive index less than
that of GMR layer 120 or 220 for total internal reflections of
light in the guided modes of GMR layer 120 or 220. For example,
substrate 130 could be a glass substrate when GMR layer 120 or 220
employs a silicon nitride waveguide. More generally, the materials
selected for GMR layer 120 or 220 and carrier substrate 130 can be
selected according to the wavelengths of light being guided.
Alternatively, the carrier substrate could be eliminated to provide
a freestanding GMR layer 120 or 220. More generally, systems 100
and 200 may be integrated onto or into essentially any surface
using many conventional manufacturing methodologies including, but
not limited to microlithography-based surface patterning and
nanolithography-based surface patterning such as used in integrated
circuit fabrication. For example, conventional semiconductor
manufacturing techniques (e.g., a CMOS compatible fabrication
process) may be employed to create a GMR grating on or in a surface
of a photonic integrated circuit (IC). In which case, system 100 or
200 may be readily integrated with conventional photonic or
electronic systems on a monolithic chip. Moreover, such an
exemplary IC-based system may have a surface footprint as small as
one square millimeter (mm) or less when fabricated using currently
available manufacturing methods.
[0016] The grating patterns and parameters of GMR layers as
described above create guided mode resonances for multiple
wavelengths .lamda..sub.1 to .lamda..sub.N that are incident at a
target angle. FIG. 3 illustrates the functional dependence of the
coupling strength of a GMR layer in accordance with an exemplary
embodiment of the invention. In particular, the coupling strength
shown in FIG. 3 has resonance peaks 310 associated with wavelengths
.lamda..sub.1, .lamda..sub.2, .lamda..sub.3, and .lamda..sub.4.
Resonance peaks 310 have widths for acceptance of light wavelengths
into the resonant mode, and such widths depend on the quality or Q
factor of the resonance. FIG. 3 shows resonance peaks 310 that have
the same quality and strength. More generally, the resonant
couplings for different wavelengths may differ in strength and
quality. For example, the properties of resonance peaks 310 for
different wavelengths may be selected to provide specific relative
Raman scattering signal strengths for different wavelengths
.lamda..sub.1, .lamda..sub.2, .lamda..sub.3, and .lamda..sub.4. In
accordance with an aspect of the invention, a grating pattern for a
GMR layer can be selected to correspond to a transform of a desired
coupling strength function having multiple separated peaks. In
particular, the resonance frequencies determine the required and
most useful grating dimensions and periodic surface structure or
structures.
[0017] FIG. 4 shows a scattering spectroscopy, chemical analysis,
or sensor system 400 in accordance with an embodiment of the
invention. As illustrated, system 400 includes a light source 410
such as a laser, a SERS/grating system 420 such as grating system
100 of FIG. 1 or grating system 200 of FIG. 2, and a detector 430.
In operation of system 400, light source 410 produces an excitation
signal 412 that illuminates system 420 at a fixed or variable
incidence angle. Excitation signal 412 has an angle of incidence
.theta. between 0.degree. and 90.degree., and the angle of
incidence is generally selected according to the target angle at
which the grating structure is designed to produce the desired
guided mode resonances. Excitation signal 412 may contain polarized
light, for example, having a transverse electric (TE) polarization
or transverse magnetic (TM) polarization. In some embodiments,
excitation signal 412 is a monochromatic collimated light beam
having a wavelength selected so that Raman scattering from a
specific analyte or specific set of analytes produces scattered
light with wavelengths .lamda..sub.1 to .lamda..sub.N.
[0018] System 420 is exposed to an analyte 422 while being
illuminated by light source 410, so that excitation signal 412
interacts with analyte 422 to produce a scattered signal
corresponding to Raman scattered photons. The scattered signal may
be enhanced by an SERS layer in system 420. System 420 also
includes a GMR layer, and the guided mode resonances established in
the GMR layer produce an evanescent electromagnetic field that can
interact with the Raman scattering process. The evanescent field
interactions thus can further enhance the scattering signal for
those specific wavelengths .lamda..sub.1 to .lamda..sub.N that
correspond to the guided mode resonances.
[0019] Light coupled into the GMR layer of system 420 is directed
to a detector 430. Detector 430 may include, for example, a
photodiode or other broadband light detector capable of measuring
the intensity of light in a wavelength range including resonance
wavelengths .lamda..sub.1 to .lamda..sub.N. Alternatively, detector
430 may include narrow band detectors capable of distinguishing
between wavelengths .lamda..sub.1 to .lamda..sub.N. Such narrow
band detectors could employ appropriate filters that select the
resonance wavelengths .lamda..sub.1 to .lamda..sub.N.
Alternatively, a dispersive element, which separates the different
wavelengths spatially, could be used, so that resonance wavelengths
.lamda..sub.1 to .lamda..sub.N can be detected simultaneously by a
position-sensitive detector, such as a tandem multichannel plate
detector. Although detector 340 is shown as a separate component in
FIG. 4, detector 430 and system 420 can alternatively be integrated
into the same monolithic chip to provide a compact chemical
sensor.
[0020] In some embodiments of system 400, SERS/grating system 420
is implemented to have guided mode resonances corresponding to
different Raman scattering wavelengths .lamda..sub.1 to
.lamda..sub.N that correspond to Raman scattering of excitation
signal 412 from one specific or targeted analyte. The detection of
multiple Raman scattered wavelengths can improve selectivity for
the targeted analyte. Further, enhancement of the Raman scattering
frequencies of the targeted analyte by interactions with the guided
mode resonances of system 420 can further improve selectivity of
detection of the targeted analyte.
[0021] In some other embodiments of system 400, SERS/grating system
420 is implemented to have guided mode resonances corresponding to
wavelengths .lamda..sub.1 to .lamda..sub.N that respectively
correspond to Raman scattering of excitation signal 412 from N
targeted analytes. System 400 can thus detect any analyte from a
set of targeted analytes. For example, in embodiment where detector
430 is a broadband detector, a warning signal may be generated when
any of the target analytes are detected, e.g., when any analyte
from a set of toxic or explosive compounds is detected. In an
embodiment where detector 430 can distinguish among wavelengths
.lamda..sub.1 to .lamda..sub.N, the specific analytes from the
targeted set can be detected or identified.
[0022] Although the invention has been described with reference to
particular embodiments, the description is only an example of the
invention's application and should not be taken as a limitation.
For example, although some of the above embodiments are described
as employing light or optical signals, those terms include
electromagnetic radiation generally and are not intended to be
limited to visible light. Various other adaptations and
combinations of features of the embodiments disclosed are within
the scope of the invention as defined by the following claims.
* * * * *